1. Field of the Invention
The invention is related to coding and decoding data, more particularly in microwave link systems.
2. Description of Related Art
The problems of prior art can be illustrated with reference to point-to-multipoint (PMP) radio systems, in which the access points (AP) operate in full-duplex mode and terminals (Access Terminal, AT) operate in half-duplex mode. FIG. 1 illustrates the structure of such a system. FIG. 1 shows terminals 10, an access point 20, and a telecommunications network 30. Typically such systems are used to provide fixed wireless connections between a central station i.e. an access point 20 (AP) and several fixed substations i.e. access terminals 10 (AT). Such systems are very advantageous in environments, where provision of fixed lines would cause prohibitive costs, such as in cities. Typically such systems are used to link base stations of a cellular telecommunications network to a central station 20, which is connected to rest of the telecommunications network 30. Such systems are also; often used for providing wireless local area networks (WLAN). Such systems are also often used to provide connections between public networks and private business and residential customers.
In many cases such systems use time division to separate signals of the terminals from each other, i.e. they are arranged to transmit at different times. For simplicity and reasons of cost, terminals typically operate in half-duplex mode, i.e. the terminals cannot transmit and receive at the same time. The access points are typically capable of full-duplex operation. The number of access points in a network is considerably lower than the number of terminals, whereby the requirements for low cost are not as stringent as in the case of terminals and the structure of access points can be more complicated.
One example of such a system is the HIPERACCESS and HIPERLAN systems specified by the European Telecommunications Standards Institute. The HIPERACCESS system is described in detail in the ETSI specification DTR/BRAN-010001 “Broadband Radio Access Networks (BRAN): Requirements and architectures for HIPERACCESS fixed networks”.
Such systems need error correction mechanisms in order to ensure that the transmitted data is correct. One typically used error correction method is Reed-Solomon coding, which is especially well suitable for correcting error bursts, i.e. errors in which a number of consecutive bits are in error. In the Reed-Solomon method, a certain number of parity symbols are calculated from the data to be transmitted and appended to the data. The data and the appended parity symbols are transmitted to a receiver, which can detect and correct errors on the basis of the information in parity symbols. However, the Reed-Solomon method has the drawback, that the receiver needs to know the number of symbols in a transmission block, i.e. the number of payload data and the number of parity symbols calculated from the payload data. This is problematic in cases, when the length of transmitted messages vary. One straightforward solution is to use a constant transmission block size which is specified to be large enough, that any possible transmitted message will fit in the block. If the message to be transmitted does not fill the whole block, the rest of the block is padded with dummy values for transmission, which dummy values are removed by the receiver from the received data. This solution has the drawback, that it creates an excessive overhead for the transmission of short messages.
These problems are typically found in transmission of broadcast messages in PMP radio link systems, in which messages the access point sends announcements and control information to the terminals. Better solutions are clearly needed.